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Originally Posted by StephenNutt
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There is no one size fits all ampacity chart, as mentioned the link above
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As you might guess, the rated ampacities are just a rule of thumb. In careful engineering the voltage drop, insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account.
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Here is a good calculator
http://www.wirebarn.com/Wire-Calculator-_ep_41.html which does allow you to include the total circuit length and desired maximum voltage drop.
Note it does not take into affect the insulation type and the CIM uses high temp insulation since it goes inside the motor where it is subject to heating caused by the motor. It is also a very short section of wire. That is why we are required to use 12ga with a CIM if we want it to be connected to a 40a breaker even if the wire that exits the CIM is 14ga.